Impact of Fishing Exploitation and Climate Change on the Grey Mullet Mugil cephalus Stock in the Taiwan Strait

Abstract

This study estimated the impact of fishing exploitation and climate change on the stock of Grey Mullet (known as Striped Mullet in North America) Mugil cephalus in the Taiwan Strait. Cohort analysis was employed to estimate the population abundance and instantaneous rate of fishing mortality by age of Grey Mullet in the Taiwan Strait. The instantaneous rate of natural mortality (M) was varied to 0.50, 0.75, 1.00, 1.25, and 1.50 times the estimated M to conduct a sensitivity analysis of varying M on correlation coefficients on other factors. Since 1986, the annual catches have sharply dropped and remained at a very low level despite high fishing intensity, indicating that overfishing has occurred since the 1980s. The population of the age-group 4 years and older has decreased since 1986, reaching 20,000 individuals in 2004, which is only 1% of what the population was in 1981. The overexploitation of Grey Mullet reduced its biomass, and environmental changes during the past two decades are likely to have driven the stock to consecutive years of poor recruitment, causing its current low level in the Taiwan Strait. We also observed that the recruitment population of Grey Mullet was influenced by the Pacific Decadal Oscillation, with the lag in its influence changing from 2-3 years from the time period of 1958-1985 to 0-2 years after 1986. In addition, sea surface temperature anomalies have shown a significant time lag of a 0-2-year effect since 1986. Overexploitation of the Grey Mullet population after the 1980s suggested an increased sensitivity to short-term environmental changes.